This NIEHS-SRP project will evaluate the biogeochemical mechanisms controlling the transport and bioavailability of DNAPL and dissolved hydrophobic compounds within groundwater and at the groundwater-surface water interface and how these processes determine the efficacy of in situ remediation methods. Lipophilic contaminants in sediments may accumulate in shellfish, posing a human health risk. Sediment remediation strategies will ideally limit contaminant transport via the aqueous phase thus limiting bioaccumulation and risk. In situ capping is one remediation technique utilized to stabilize and physically isolate contaminated sediments to reduce exposure to biota and humans. Caps are designed to resist physical disturbance by erosion and bioturbation, and are consequently composed primarily of coarse granular material, which is highly permeable. Recent work has shown that groundwater-surface water interactions include numerous advection mechanisms that operate independently of canonical groundwater flow. As a result, DNAPL and contaminated porewater advection may be orders of magnitude higher than expected from groundwater considerations alone, reducing the effectiveness of capping for in situ remediation. This study will provide new understanding of surface water- groundwater interactions and DNAPL contaminant degradation, transport, and bioavailability; novel methods for evaluating shallow advection rates and pore water PAH concentrations for site characterization to guide in situ remediation; and guidance for remediation managers to improve safe and effective cap design where groundwater advection processes are active. Hypothesis 1. Bioavailable PAH concentrations in porewater at the sediment-water interface are decoupled from PAH flux due to non-equilibrium effects when advection dominates diffusional transport. 2. DNAPL mobility is enhanced by the intrusion of dense seawater, resulting in transport of liquid organic phase in saline and brackish coastal groundwater. 3. Advection in subaqueous permeable sediments results in measurable disequilibrium between parent and daughter nuclides of the naturally-occurring Th-Ra-Rn system that is proportional to advective flux. 4. Advection dynamics and seawater intrusion increase bioavailable PAH flux and NAPL transport in permeable cap materials used for in situ remediation at contaminated sites in the Elizabeth River.